Apple logo keeps flashing on and off

An Error 9 message on an iPhone 6 can be a lot of things but pretty much all of them require highly advanced repairs. The immediate DIY thing to try is to use different and known good cables and computers. Once that is ruled out, you can try different software tools, such as 3uTools but I suspect you will get the same problem.

One thing you could try is to apply pressure on the CPU with your thumb while it is booting. If that works, then you will need to find someone who has mastered the Error 9/CPU Grind repair. Otherwise it could be a NAND issue which would require a reballing and reprogramming to fix data corruption. It could also be Tristar related but usually it is NAND/CPU.